The present invention concerns compounds that are useful for dispersing pigments in electrocoat coating compositions. In particular, the dispersant compounds of the invention may be used to disperse organic or inorganic pigments in the electrocoat compositions. The invention also provides a method for coating substrates using the inventive electrocoat compositions and the coated articles so formed.
Electrocoating, or electrodeposition coating, is widely used in the art for the application of polymeric coatings to electrically conductive substrates. In the electrocoat process, the substrate to be coated is used as one electrode in an electrical cell so that a current passed through the aqueous bath will deposit the coating onto the substrate. One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer over all portions of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat baths usually comprise an aqueous dispersion of a principal film-forming resin, such as an acrylic or epoxy resin, having ionic stabilization. For automotive or industrial applications where hard electrocoat films are desired, the electrocoat coating compositions are formulated to be curable compositions. Usually, this is accomplished by including in the bath a crosslinking agent that can react with functional groups on the principal resin under appropriate conditions (e.g., with the application of heat) and thus cure the coating. Alternatively, the principal resin may be a self-crosslinking resin, having on the same resin one or more kinds of groups that are reactive toward each under the appropriate curing conditions.
Pigments are included in the composition for decorative effect and also, in the common situation where the electrocoat is used as a primer, to enhance the corrosion protection properties of the coating. In either case, it is essential that the pigment be well-dispersed in the electrocoat bath in order to promote bath stability, as well as adequate and uniform dispersion of the pigment in the deposited and subsequently cured coating. It is desirable to have the pigment dispersant molecules cure into the film to develop optimum properties in the cured coating. For this to happen, the dispersant molecule must carry one or more groups reactive toward either the principal resin or, if included, the crosslinking agent. When the dispersant molecule is reacted into the cured film it is possible to thereby increase the integrity of the film, as well as to prevent the dispersant from acting as a water-sensitive plasticizer in the cured film.
The dispersant compound of the invention has a polymeric acrylic backbone on which are a pigment-interactive substituent and a stabilizing substituent. The stabilizing substituent provides nonionic stabilization in the electrocoat composition. Until now, dispersants have usually been only ionically stabilized. In addition, such dispersants usually have been either monomeric or epoxy-based compounds. Such dispersants are described, for example, in U.S. Pat. No. 3,947,339 (describing a cationic pigment dispersant that is the reaction product of stearyl glycidyl ether and N-methyl ethanolamine; a cationic resin for pigment dispersion that is the reaction product of a polyglycidyl ether of Bisphenol A with epoxy equivalent weight of 500 and diethylamine; and an alkyl imidazoline as a pigment dispersant); U.S. Pat. No. 5,281,316 (describing a pigment grinding vehicle prepared from a diglycidyl ether of Bisphenol A with epoxy equivalent weight of about 200, Bisphenol A, 2-ethylhexanol-half-capped toluene diisocyanate, and a quaternizing agent prepared by reacting dimethylethanolamine with 2-ethylhexanol-half-capped toluene diisocyanate); U.S. Pat. No. 4,443,569 (describing an epoxy-acrylic graft copolymer used to disperse pigments); and U.S. Pat. No. 4,769,400 (describing a grind resin prepared by reacting an epoxy resin based on bisphenol A, having an epoxy equivalent weight of 890, with diethanolamine).
Such methods often require lengthy processing times and are particularly ineffective for dispersing organic pigments in the electrocoat bath. Compared to the dispersions of the invention, the prior art dispersions have relatively poor color development and stability. Also, the ratio of pigment solids to resin solids by weight (often referred to as the pigment to binder ratio) is often relatively low for the prior art pigment paste dispersions. A higher pigment to binder ratio is desirable because it increases manufacturing efficiency. Furthermore, the compounds of this invention provide significant and unexpected advantages in reduced milling times and greater formulating latitude when the pigment paste dispersions are incorporated into paint compositions.
In U.S. Pat. No. 5,231,134, Carpenter et al. describe a pigment dispersant for cathodic electrocoating compositions that is prepared by polymerizing an ethylenically unsaturated monomer containing an isocyanate group with other ethylenically unsaturated monomers, and reacting the isocyanate group stepwise or simultaneously with a polyalkylene glycol monoalkyl ether and/or an amino-terminated polyalkylene glycol monoalkyl ether, and a compound containing at least a tertiary amine group and one functional group capable of reacting with the isocyanate group. The invention of the patent, like the present invention, provides good pigment pastes or pigment dispersions that, when used in an electrocoat bath, have less pigment settling and allow for use of a reduced amount of pigment and a reduced amount of volatile organics.
However, the present invention has an advantage over the prior dispersant of Carpenter et al. because the present invention can be reacted into the film when the coating is deposited on the substrate and cured. Reaction of the pigment dispersant into the film is desired in order to obtain optimum film integrity, physical properties, and durability. Moreover, organic pigments are often surface-treated to have anionic groups. The present invention has the capability of not only associating ionically with the pigment in the electrocoat bath, but also of binding the anionic treated pigment particles into the film during the curing process.